1. Field of the Invention
The present invention generally relates to a method and apparatus for x-ray analysis of particle size, XAPS for short, and more specifically to a method and apparatus for determining particle size and particle size distribution of crystalline particles comprising powders, suspensions or solids non-intrusively without the need to extractor separate the particles from the other ingredients of the materials.
2. Related Art
The overwhelming majority of materials handled by industry, such as mining, chemicals, construction, agriculture and waste products, are in particle or xe2x80x9cpowderxe2x80x9d form. Many technologically important materials such as ceramics, metals, composites, solid propellants, catalysts, magnets and high-TC superconductors all constitute particles or xe2x80x9cgrainsxe2x80x9d and are manufactured by consolidation of powders. Particle size is one dominant parameter in all these industrial products that dictates their properties and performance. Determination and control of particle characteristics, especially the particle size distribution, are essential for product quality control and performance.
Various methods have been developed in the past to determine the particle size distribution in powders. These range from sieve elimination to laser scattering. Each one of these techniques has its unique advantage and limitations.
The early techniques for characterizing fine particles depended heavily on sieves, elutriators and microscopes. These techniques are time consuming and do not lend themselves to fast and practical measurements. In the period from the mid-1950""s to the mid-1970""s the methodology of fine particle characterization improved rapidly with the introduction of the instruments as the Coulter Counter, other resista-zone counters and image analyzers. Since mid-seventies the fine particle characterization studies have expanded substantially and many new techniques and instrumentation have been developed. The highlights of this era include holography for characterization of particles in mist and suspension systems; laser Doppler velocimetry (laser-photon correlation spectroscopy) for characterization of particles in aerosols and Brownian motion; eriometry (light/laser diffraction) for evaluating fine particle populations based upon group diffraction patterns; signature-waveform characterization of scattered light for fine particle analysis; fractal description of fine particle profiles; and a new generation of image analyzers with powerful digitization and computer routines for fine particle size and shape analysis.
Most of the recent techniques for particle size determination are based on indirect measurements such as the optical properties of particles obtained from scattering, diffraction, etc., of light or laser directed at the particle surface, or the disturbance of a homogeneous electrical field by a passing particle. If an irregular shaped particle is measured using these physical properties, the xe2x80x9csizexe2x80x9dof this particle will differ and depend on the particular property chosen. In these techniques, particle size is described by its so-called equivalent diameter, the diameter of a sphere, which yields the same response when analyzing a certain property as the irregularly shaped particle. For these reasons significant differences are found in the particle size distribution results obtained by different commercially available instruments. For measurement of particle size in loose powders the scanning electron microscope (SEM) is a very useful tool because of its superior depth-of-focus compared to optical microscopes. However, use of SEM is extremely time consuming in order to obtain statistically significant measurements. It also needs to operate under vacuum and is not amenable for on-line applications.
On the other hand, none of the current particle-size analysis techniques is applicable to multi-particle mixed solid materials, except for microscopy in certain cases. Microscopy, however, requires destructive sectioning of the solids followed by tedious polishing and etching procedures. These procedures are difficult and time consuming, and sometimes unsuccessful for many ceramics, intermetallics, composites, energetics, and some metals. Particle size analysis of fillers in viscous suspensions (uncured) where the particles are encapsulated is yet another area, which is not feasible even with microscopy.
Analysis of particles in some of the suspensions and solids by these techniques might be feasible only after their constituents are separated effectively. One such technique involves the separation of particles, e.g. separation of solid filler particles from a suspension by heating in an oven to pyrolyze and eliminate the viscous phase. Thereafter, the remaining particulate can be characterized by the known methods. Such intrusive approaches, however, are usually ineffective and expensive.
All the methods mentioned so far, including the early methods, do not provide information on the constitution of the fine particles, i.e., when the fine particles contain more than one material or phase-polymorph, they are not differentiated by these techniques. Scanning electron microscopy (SEM) combined with energy dispersive x-ray fluorescence analysis (EDX) can differentiate compositional differences between the particles in a mixed material. However, SEM with EDX is applicable in general only if the components contain different and contrasting elements that are heavier than oxygen and are not affected by the vacuum. The EDX technique is also limited to submicron thick surface layers and prone to errors due to surface films. Use of SEM with EDX is time consuming and is not amenable for on-line applications.
X-ray diffraction methods can be applied to determine the size of particles in some special cases. Early work has been done with Debye and back-reflection cameras. In these x-ray diffraction techniques particles or grains of a polycrystalline material are irradiated with a collimated beam and diffraction takes place in the coherently reflecting planes of the particles. When large numbers of particles are irradiated under the incident beam, their diffraction spots overlap and form continuous diffraction lines on appropriate Debye rings. Continuity of the rings breakdown and individual diffraction spots are resolved if the number of diffraction particles is reduced. However, the number of diffracting particles is reduced and diffraction spots from individual particles are resolved only if the particle size is very large. Furthermore, these x-ray techniques are very tedious and cannot be applied routinely.
Previous efforts in this area include:
Mack, U.S. Pat. No. 3,148,275 discloses a x-ray technique that is not for particle size analysis. Rather, it relates to a special sample holder to hold a curved specimen to improve wide-angle x-ray diffractometer (WAXRD).
Goebel, U.S. Pat. No. 4,144,450 does not disclose a particle size analyzer, but rather relates to a wide-angle x-ray powder diffractometer equipped with a horizontal linear position sensitive proportional counter (PSPC) for simultaneous data collection from a range of 2xcex8 angles, on the equatorial diffraction plane. This is a regular WAXRD technique with a horizontal linear position-sensitive detector (PSD). This is not a particle size analyzer.
Ladell, U.S. Pat. No. 4,199,678 does not disclose a particle size analyzer. Rather, it relates to a modified WAXRD for texture (preferred orientation) analysis with a point detector.
Rinik. et al., U.S. Pat. No. 4,649,556 discloses an indirect method to get information on the xe2x80x9caveragexe2x80x9d particle size by making use of the variation of diffracted intensity with WAXRD 2xcex8 angle using a point detector. It does not obtain direct information on the particle size and cannot do measurements on individual particles to get particle size distribution.
Cocks. et al., U.S. Pat. No. 4,821,301 discloses a technique for glancing-angle x-ray-absorbance chemical analyses of thin (1000 xc3x85) films. It does not relate to particle size analysis.
Moulai, U.S. Pat. No. 5,128,976 does not disclose a particle size analyzer. Rather, it relates to an oscillation radiographer with a point detector. It is based on absorption contrast and uses a x-ray film to record it. It does not use any of the beam path on the detector system nor the type of data analysis that the present invention (XAPS) uses.
Goebel, U.S. Pat. No. 5,373,544 does not disclose a particle size analyzer. Rather, it relates to an optimized WAXRD designed for the capillary samples. It utilizes a curved mirror to focus the primary x-ray beam and a mobile horizontal linear position sensitive proportional counter with a radial collimator for simultaneous data collection from a range of 2xcex8 angles on the equatorial diffraction plane.
Carpenter, U.S. Pat. No. 5,418,828 does not disclose a particle size analyzer like the present invention disclosed here, where large number of particles, typically 0.51 xcexcm to 300xcexcm in size, can quantitatively be analyzed simultaneously. This technique, rather, is meant for particle imaging for particles 1-2 mm in diameter or larger and generally will not work for powders with smaller particle sizes. It uses a linear position sensitive detector in horizontal configuration as opposed to a vertical configuration of the present invention (XAPS). It uses the diffraction information by scanning to construct a low resolution xe2x80x9cimagexe2x80x9d of a very large particle at one angular setting. In the present invention, diffraction information is obtained from rocking the particles to get the total integrated intensity, which corresponds to total volume/size of the particles, and it is the xe2x80x9cintegrated intensityxe2x80x9d not the xe2x80x9cimagexe2x80x9d that is utilized for particle size analysis.
Hautman, U.S. Pat. No. 5,446,777 discloses WAXRD with a horizontal linear position-sensitive detector that is designed to carry out locationxe2x80x94specific WAXRD measurements on a give sample. It does not relate to a particle size analyzer.
Yazici, et al., xe2x80x9cDefect Structure Analysis of Polycrystalline Materials by Computer-Controlled Double-Crystal Diffractometer with Position-Sensitive Detector,xe2x80x9d J. Appl. Cryst. (1983), discloses a computerized double-crystal diffractometer and a position-sensitive detector which analyzes defects in solid specimens.
None of these previous efforts, taken alone or in combination, teach or suggest all of the elements of the present invention, nor do they disclose the advantages of the present invention.
It is a primary object of the present invention to provide a method and apparatus for detennining particle size and particle size distribution of crystalline particles in powders, suspensions and solids non-intrusively without the need to extractor separate the particles from the rest of the material.
It is another object of the present invention to provide a method and apparatus for determining the particle size at rates near real time for on-line process/product quality control applications in various manufacturing operations.
It is another object of the present invention to provide method and apparatus, which can determine particle size distribution of the solid ingredients of formulations involving a plurality of different types of crystalline particles.
It is another object of the present invention to provide a method and apparatus, which can determine particle size distribution of particles in more than one phase or polyrnorph.
It is another object of the present invention to provide a method and apparatus for differentiating between different components of a composite or mixture in determining particle size distribution.
It is another object of the present invention to provide an apparatus for determining the particle size distribution, which includes an x-ray source, a monochromator, a position sensitive detector and computer means for determining particle size distribution.
It is still even another object of the present invention to provide a method and apparatus for determining particle size distribution which rocks a specimen or the x-ray source through the angular range of reflection of the particles at Debye arc or portion thereof.
These and other objectives are achieved by the apparatus of the present invention, which comprises a x-ray source, a monochromator, a goniometer, a position sensitive detector and computer means for interpreting the data obtained at the position sensitive detector. The method of the present invention includes the steps of generating an x-ray; narrowing the wavelength of the x-ray by means of a monochromator; placing a specimen in the path of the x-ray beam; allowing the particles to diffract the beam; detecting the diffracted beam with a position sensitive detector; collecting the diffraction data from individual particles; rocking or rotating the specimen or the x-ray source for successive times to cover the angular range of reflection of the particles; compilation of the diffraction data in the computer memory to construct the intensity profile for individual particles; and interpreting the data to determine particle size and distribution of crystalline particles.
By the method and apparatus of the present invention, particle size and particle size distribution of crystalline particles in powders, suspensions and solids can be determined upon collection of samples from a process and characterization off-line at another location with the apparatus and method of the present invention, or on-line with the process using the apparatus and the method of the present invention. Importantly, the present invention allows for these determinations to be made in situ, without the need for separating the particles, and sufficiently fast so that the generated data can be used in a process control algorithm for quality and process control.